1. Technical Field
The present application relates to a method of producing a porous silica material.
2. Description of the Related Art
A low-density porous silica material, generally referred to as silica aerogel, has many fine pores and is formed of voids by 90% or more of its volume. In addition, its skeleton is formed of spherical silica fine particles each having a size of about several nanometers to several tens of nanometers that are connected together. The porous silica material has a low density and a low refractive index. In addition, a speed of sound propagating in the porous silica material (hereinafter referred to as sound speed of the porous silica material) is lower than the sound speed in the air, i.e., 340 m/s. Accordingly, the porous silica material has been attracting attention as an acoustic propagation medium for use in various acoustic devices. For example, Non-patent Document No. 1 (Hidetomo Nagahara, Takashi Hashida, Masa-aki Suzuki, Masahiko Hashimoto, “Development of High-Sensitivity Ultrasonic Transducer in Air with Nanofoam Material,” Japanese Journal of Applied Physics, Vol. 44, No. 6B, pp. 4485-4489, 2005) discloses that, through utilization of such property of the porous silica material as to have a low sound speed, the porous silica material is useful as an acoustic matching layer for efficiently incorporating sound waves in the air into an ultrasonic transducer.
In order to improve performance of the porous silica material as the acoustic matching layer, it is necessary to reduce an acoustic impedance of the porous silica material. For this purpose, it is necessary to further reduce the sound speed of the porous silica material. Non-patent Document No. 1 discloses a relationship between a density and sound speed of the porous silica material. As shown in FIG. 10, when the density is reduced, the sound speed also reduces. According to FIG. 10, when the density is reduced to about 70 kg/cm3, the sound speed reduces to about 75 m/s.
Non-patent Document No. 2 (J. Fricke, T. Tillotson “Aerogels: production, characterization, and applications” Thin Solid Films 297 (1997) 212-223) also discloses, as shown in FIG. 11, a relationship between a density and sound speed of the porous silica material. When the porous silica material is placed in the atmosphere, in the case where the density falls within the range of more than about 50 kg/cm3, the sound speed reduces when the density is reduced, as in FIG. 10. However, in the case where the density falls within the range of less than about 50 kg/cm3, the sound speed starts increasing when the density is reduced. The sound speed when the porous silica material is placed in a vacuum reduces with density, and hence the fact that the sound speed starts increasing in the atmosphere is probably attributable to an influence of the air. According to FIG. 11, when the porous silica material is placed in the atmosphere, the lowest sound speed is about 100 m/s.
The porous silica material is synthesized with use of a silicon alkoxide (alkoxysilane) as a raw material. Tetramethoxysilane (hereinafter abbreviated as TMOS) represented by the following chemical formula (1) is generally used as the silicon alkoxide. First, TMOS is mixed with a solvent such as ethanol to manufacture a raw material solution.

Next, an aqueous solution containing a catalyst is added to the raw material solution to cause hydrolysis and condensation polymerization reactions, to thereby generate a wet gel. After that, a liquid in the wet gel is replaced with a gas (dried). Thus, the porous silica material is obtained.
When the liquid in the wet gel is replaced with a gas, if a tensile stress based on a surface tension of the liquid remaining in the fine pores exceeds strength of the gel, its gel structure is destroyed. To prevent this, supercritical drying is often used in a step of drying the wet gel. In addition, a porous silica material that degrades with time to only an extremely small degree can be obtained by subjecting the gel to hydrophobizing treatment.